Investment casting compositions, molds and related methods

ABSTRACT

Various embodiments of a slurry composition for investment casting and a method of forming such composition are disclosed. The slurry composition can include a refractory material, a binder, a solvent, and a thixotropic agent that includes fibrillated fibers. The slurry composition can also include a filler that includes glass bubbles.

BACKGROUND

Investment casting, sometimes referred to as a “lost wax” process, is a well-known method of manufacturing components having intricate and complex shapes. This process is used in diverse large- and small-scale applications, ranging from the manufacture of superalloy gas turbine engine components to tiny customized orthodontic appliances.

An investment casting process typically begins with the preparation of a sacrificial wax pattern having a size and shape similar to that of the device to be manufactured. This wax pattern can be made by molding, a rapid prototyping process, or any other method. The pattern then undergoes a shelling process in which it is sequentially dipped into tanks containing coating materials, typically ceramic slurries. Each layer of coating material is given time to dry before the next dip. Additionally, dry refractory granules, or stucco, can be applied between dips to enhance the structural integrity of the shell. This process can be repeated over and over to gradually build up a shell having multiple ceramic layers.

After the shell is thus formed, the pattern is heated, typically using a flash furnace or steam autoclave, to melt the wax and allow it to be extracted from the mold. The end result is a mold with a hollow cavity faithfully reproducing the shape of the pattern. At this point, the mold can be further strengthened by firing. A molten metal alloy can then be introduced into the mold cavity to cast the desired part. Finally, after the alloy has been sufficiently cooled, the mold can be mechanically or chemically disintegrated to separate the cast part from the mold.

In conventional investment casting methods, the finished shell contains six or more layers, each of which could include two or more sub-layers of slurry or stucco. The first layer, known as a prime coat, is applied directly to the wax pattern. The prime coat often includes both a refractory slurry and a refractory stucco. The finished shell can include one or more prime coat layers. The next layer, known as the intermediate coat, is applied over the prime coat and also includes a refractory slurry and a refractory stucco. As with the prime coat, the finished shell can also include one or more intermediate layers. Following application of the prime and intermediate coats, three or more backup coats are generally applied to build up the thickness of the shell. Each backup coat also commonly includes a refractory slurry and a refractory stucco. In many cases, a final seal coat is then applied over the final backup coat to prevent stucco from coming loose from the shell during further processing of the shell.

SUMMARY

In general, the present disclosure provides various embodiments of a slurry composition for investment casting and a method of forming such composition. The slurry composition can include at least one of a refractory material, a binder, a solvent, and a thixotropic agent. In one or more embodiments, the thixotropic agent can include fibers, e.g., fibrillated fibers. Further, in one or more embodiments, the slurry composition can also include a filler that includes glass bubbles.

In one aspect, the present disclosure provides a slurry composition for investment casting that includes a refractory material, a binder, a solvent, and a thixotropic agent that includes fibrillated fibers.

In another aspect, the present disclosure provides a method of making an investment casting mold. The method includes coating a sacrificial pattern with a prime layer including a first refractory slurry and a first refractory stucco, at least partially hardening the prime layer, and coating the prime layer with an intermediate layer that includes a second refractory slurry and a second refractory stucco. The method further includes at least partially hardening the intermediate layer, coating the intermediate layer with a backup layer that includes a thixotropic agent that includes fibrillated fibers, and at least partially hardening the backup layer.

In another aspect, the present disclosure provides a slurry composition for investment casting that includes a refractory material, glass bubbles, and a thixotropic agent that includes fibrillated fibers.

In another aspect, the present disclosure provides a method of forming a slurry composition for investment casting. The method includes providing a dry composition that includes a refractory material, glass bubbles, and a thixotropic agent that includes fibrillated fibers; and combining the dry composition with at least one of a binder and a solvent to form the slurry composition.

In another aspect, the present disclosure provides an investment casting mold. The mold includes a prime layer, an intermediate layer disposed on the prime layer, and a backup layer disposed on the intermediate layer. The mold further includes a seal layer disposed on the backup layer. At least one of the prime layer, intermediate layer, backup layer, and seal layer includes a slurry composition including a refractory material and a thixotropic agent that includes fibrillated fibers.

These and other aspects of the present disclosure will be apparent from the detailed description below. In no event, however, should the above summaries be construed as limitations on the claimed subject matter, which subject matter is defined solely by the attached claims, as may be amended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section view of one embodiment of a multilayered investment casting mold.

FIG. 2 is an enlarged fragmentary cross-section view of an inset portion of the investment casting mold of FIG. 1.

FIG. 3 is a schematic cross-section view of another embodiment of a multilayered investment casting mold.

FIG. 4 is a schematic cross-section view of another embodiment of a multilayered investment casting mold.

FIG. 5 is a schematic cross-section view of another embodiment of a multilayered investment casting mold.

FIG. 6A is an optical micrograph of a cross-section of an investment casting mold of Comparative Example B.

FIG. 6B is an optical micrograph of a cross-section of an investment casting mold of Example 2.

FIG. 7 is a plot of experimental data from Comparative Example B and Example 2 showing slurry shear stress versus shear rate.

FIG. 8A is an optical micrograph of a cross-section of an investment casting mold of Comparative Example C.

FIG. 8B is an optical micrograph of a cross-section of an investment casting mold of Example 3.

FIG. 8C is an optical micrograph of a cross-section of an investment casting mold of Example 4.

FIG. 9A is a plot of experimental data from Comparative Example C, Example 3, and Example 4 showing slurry weight versus time after a first coat has been applied to a wax plate.

FIG. 9B is a plot of experimental data from Comparative Example C, Example 3, and Example 4 showing slurry weight versus time after a fifth coat has been applied to the wax plate.

FIG. 10A is a bar chart of weights of slurry, stucco, and total weight of the coating retained on a wax plate after a first dip for Comparative Example C, Example 3, and Example 4.

FIG. 10B is a bar chart of weights of slurry, stucco, and total weight of the coating retained on the wax plate after a third dip for Comparative Example C, Example 3, and Example 4.

FIG. 10C is a bar chart of weights of slurry, stucco, and total weight of the coating retained on the wax plate after a fifth dip for Comparative Example C, Example 3, and Example 4.

DEFINITIONS

As used herein:

“refractory” refers to a heat-resistant ceramic material;

“slurry” refers to a fluid mixture of a solid grain with a liquid;

“stucco” refers to a solid grain having a particle size usually not typically coarser than a U.S. sieve 30 mesh screen;

“thixotropic” refers to a shear-thinning property, where a gel or liquid becomes less viscous when it is shaken, agitated, or otherwise stressed;

“wax” refers to a polymeric substance capable of melting at low temperatures to yield a low viscosity liquid; and

“zircon” refers to zirconium silicate, having the chemical formula ZrSiO₄.

As used herein, the terms “preferred” and “preferably” refer to embodiments described herein that may afford certain benefits under certain circumstances. Other embodiments may also be preferred, under the same or other circumstances. Further, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” or “the” component may include one or more of the components and equivalents thereof known to those skilled in the art. Further, the term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements.

It is noted that the term “comprises” and variations thereof do not have a limiting meaning where these terms appear in the accompanying description. Moreover, “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably herein.

Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.

DETAILED DESCRIPTION

The present disclosure describes, by way of illustration and example, slurry compositions used to produce investment casting molds. The illustrated patterns and associated sprues are exemplary, not drawn to scale, and may differ widely in size and shape depending on the application at hand. It is further understood that the refractory materials, solvents, and binders herein described are exemplary and may be substituted or modified according to the knowledge of one skilled in the art.

While the compositions and related methods described herein enable one of skill in the art to make and use investment casting molds with certain advantageous properties, it is appreciated that these compositions and methods could be further combined with additives or enhancements not examined here. For example, slurry compositions could further include gaseous or solvent-based gelling agents, chemically treated refractory materials, and slurry binder systems that interact with one another. In one or more embodiments, other additives such as microsilicas and pozzolans can also be utilized.

Creating the aforementioned layers of the shell involves a substantial amount of time. Substantial amounts of time are involved not only in the dipping process used to apply each of the constituent slurry and/or stucco layers, but also the drying steps that follow the coating of each major layer. The large number of steps in the manufacturing process also heightens the overall risk of inadvertently inducing a defect or causing damage to the shell.

In general, the present disclosure provides various embodiments of a slurry composition for investment casting and a method of forming such composition. The slurry composition can include at least one of a refractory material, a binder, a solvent, and a thixotropic agent. In one or more embodiments, the thixotropic agent can include fibers, e.g., fibrillated fibers. Further, in one or more embodiments, the slurry composition can also include a filler that includes glass bubbles.

One or more embodiments of the present disclosure can provide various advantages. For example, one or more embodiments of investment casting slurries or compositions described herein can be a dry composition such that the slurry can be more easily transported. One or more solvents and other liquid compositions can be added to the dry composition to provide the final slurry. Further, one or more embodiments of slurry compositions described herein can provide shorter drain times and better shell build, which can increase foundry shell room throughput and better shell yield while potentially reducing a number of dips required to build sufficient shell thickness to retain the molten metal during casting. Further, the addition of fibrillated fibers to slurry compositions can increase the ability of the water to be wicked from the surface of the shell, thereby reducing dry times required between dips. Investment casting molds that include fibrillated fibers can also exhibit improved toughness (i.e., ductility). In embodiments of slurries that include glass bubbles, the resulting investment casting molds can be lighter in weight and have improved crack-tip strength. The glass bubbles can, in one or more embodiments, act as a flow aid that improves flowability of the slurries.

FIG. 1 is a schematic cross-section view of one embodiment of an investment casting mold 100. The mold 100 is shown encapsulating a substantial portion of a sacrificial pattern 102, which has a tree-like structure with a centrally located trunk 103 and a plurality of branches 105 extending outwardly from the trunk 103. The pattern 102 is exemplary and there are no particular restrictions on its size or shape.

In one or more embodiments, the pattern 102 is made from wax, polymer resin, or other suitable pattern material capable of being subsequently melted, vaporized, burned, or dissolved to leave behind, with minimal residue, a cavity conforming to the exterior contours of the pattern 102.

As shown, the mold 100 includes a series of successive layers built up by dipping the pattern 102 into containers of refractory slurry or slurry composition. After withdrawing the pattern 102 following each dip, excess slurry/stucco is allowed to drain off. Optionally, the pattern 102 is manipulated by hand or mechanically to promote uniform coverage. Refractory granules, or stucco, are then applied to the wet slurry coating. Here, the combination of slurry and stucco includes a single major layer, which then is allowed to dry and at least partially harden before the next coat is applied. By repeating this process, the walls of the mold 100 are progressively built up, layer upon layer, until the overall mold 100 has the strength to withstand the physical handling forces induced by metal casting.

Beginning from the innermost layer and ending with the outermost layer, mold 100 includes a prime layer 104, an intermediate layer 110, a first backup layer 116, a second backup layer 122, a third backup layer 128, and a seal layer 134.

While the mold 100 of FIG. 1 represents a six-layered construction, additional or fewer layers may also be used depending on the nature of the application. For example, factors such as the molten metal head pressure and the size of the casting to be poured from the final mold can influence the number of backup layers used. Common commercial investment casting shells often use four backup layers. Any suitable number of backup layers can be utilized to form the mold 100.

Each of the six layers enumerated herein are described in further detail in reference to the inset, FIG. 2. The prime layer 104 is an innermost layer extending across and contacting the pattern 102. The prime layer 104 is intended to come into direct contact with molten metal after the finished mold 100 has been de-waxed and fired. As shown, the prime layer 104 includes two sub-layers—an inner layer of refractory slurry 106 and an outer layer of refractory stucco 108. In one or more embodiments, both the refractory slurry 106 and refractory stucco 108 include zircon particles (shown here as round particles) although this need not be the case. In one or more embodiments, one or more additional prime layers may be used. This may be the case, for example, where there is no intermediate slurry layer capability.

The intermediate layer 110, and successive backup layers 116, 122, 128 also include two sub-layers each, i.e., a layer of refractory slurry 112, 118, 124, 130 and an adjacent layer of refractory stucco 114, 120, 126, 132, respectively. The refractory slurry 112, 118, 124, 130 can include any suitable slurry composition. In one or more embodiments, the slurry composition includes a refractory material, a binder, a solvent, and a thixotropic agent. In one or more embodiments, the thixotropic agent includes fibers, e.g., fibrillated fibers. While the slurry composition is described regarding the backup layers 116, 122, 128, such slurry composition can be utilized for any suitable layer used to form the mold 100, e.g., one or more of the prime layer, the intermediate layer, and the seal layer. The refractory stucco (represented in the figures as jagged-edged particles) may include a fused silica, alumino-silicate, zircon, aluminum oxide, or mixture thereof. The stucco can be applied either by sprinkling it onto a freshly coated slurry by hand or by rainfall sander, or by immersion into a fluidized bed of stucco. In one or more embodiments, the size of the stucco particles generally increases from the inside to the outside of the mold 100.

Optionally and as shown, a seal layer 134 is located on the outermost periphery of the mold 100. The seal layer 134 serves the purpose of preventing stucco from the backup layer 128 from coming loose during subsequent processing of the finished mold 100 and can have a composition identical or similar to that of the intermediate or backup slurries. In exemplary embodiments, the seal layer 134 contains a fused silica, alumino-silicate, zircon, aluminum oxide, or a mixture thereof.

In an exemplary method, the resulting structure as shown in FIGS. 1-2 can then be fully dried and heated to melt the pattern 102 and remove the pattern 102 from the finished investment casting mold 100. To add greater strength, the finished mold 100 can be fired in a curing oven at temperatures of about 980 degrees Celsius.

As mentioned herein, the slurry composition utilized to form one or more layers of the mold 100 includes a refractory material, a binder, a solvent, and a thixotropic agent. In one or more embodiments, the thixotropic agent includes fibers, e.g., fibrillated fibers.

The refractory material (i.e., refractory flour or powder) is a first major component of the slurry composition. Refractory powders commonly used in the investment casting industry are zircon (ZrSiO₄), silica (SiO₂), both fused and quartz, alumina (Al₂O₃), zirconia (ZrO₂), and alumino-silicate (various combinations of Al₂O₃ and SiO₂, commonly fired at high temperatures). Refractory materials usable in the slurry and/or stucco can include fused silica, alumino-silicate, zircon, aluminum oxide and mixtures thereof. Although not critical, the refractory powder can have a wide particle size distribution, including sizes as large as 30 mesh along with sub-micron particle sizes.

The binder is a second major component of the refractory slurry. For the purposes described herein, the binder may include a refractory binder, an organic binder, or a combination of both. Refractory binders that may be contained in the refractory slurry include a variety of ceramic materials, including silicates, alkali metal silicates, silica sols, aluminum oxychloride, aluminum phosphate, gypsum-silica mixes, cements, tetraethyl orthosilicates (TEOS), and mixtures thereof. In one or more embodiments, the refractory binder includes colloidal silica. Organic binders can be thermally decomposable and include polyvinyl alcohol, polyvinyl butyral, methyl cellulose, carboxymethyl cellulose, ethyl cellulose, and mixtures thereof. Exemplary binders are described, for example, in U.S. Pat. No. 3,165,799 (Watts), U.S. Pat. No. 3,903,950 (Lirones), U.S. Pat. No. 5,021,213 (Kato et al.), and U.S. Pat. No. 6,020,415 (Guerra). Alternatively, the organic binder could include a mixture of colloidal sol and at least one acrylic latex polymer. The colloidal sol could be, for example, a silica sol, zirconia sol, alumina sol, or yttria sol, while the latex polymer could be an acrylic latex polymer, acrylic polymer, styrene-butadiene latex polymer, or a mixture thereof.

The solvent is generally the same as the liquid dispersant used for the binder. In the presently exemplary embodiments, water is the preferred solvent. Many other solvents are available, however, including other polar solvents such as mineral acids, alcohols such as methanol, ethanol, isopropanol, and butanol, glycols and glycol ethers, and mixtures thereof. Commercial binders are often provided in solution form, so the step of adding of a separate solvent may not be necessary.

The composition of the slurry composition further includes the thixotropic agent (or shear-thinning agent). Any suitable thixotropic agent can be utilized. In one or more embodiments, the thixotropic agent includes one or more fibers. Any suitable fibers can be utilized. For example, in one or more embodiments, the fibers can include fibrillated fibers. As used herein, the term “fibrillated fiber” means a fiber that is a multifilament yarn-like strand having interconnecting, fibrous elements that intermittently unite and separate at irregular intervals through one or more of the width, length, and thickness of the strand. Any suitable fibrillated fibers can be utilized for the slurry composition. Although not wishing to be bound by any particular theory, the fibrillated fibers can provide various thixotropic properties to the slurry composition.

In one or more embodiments, the fibrillated fibers include organic fibers, e.g., fibers that include at least one of high density polyethylene, polypropylene, fluoropolymers, meta aramid, para aramid, spandex, elastane, ultrahigh molecular weight fibers, wood pulp, and combinations thereof.

In one or more embodiments, the fibrillated fibers can include inorganic fibers, e.g., fibers that include at least one of glass, mineral, polycrystalline ceramic fibers such as alumina and alumina silicate (e.g., 3M™ Nextel textiles available from 3M Company, St. Paul, Minn.), and combinations thereof.

In one or more embodiments, the fibrillated fibers can include both organic and inorganic fibers.

The fibers can be present in the slurry composition in any suitable amount. In one or more embodiments, the fibers present in the slurry composition can be present in an amount of at least 0.005 weight percent and no greater than 1.5 weight percent, based on the overall weight of the composition.

The fibers in the slurry composition can have any suitable dimensions. In one or more embodiments, one or more fibers in the refractory slurry can have an average diameter of at least 1 micron and no greater than 10 microns. Further, the fibers in the slurry composition can have any suitable length. In one or more embodiments, one or more fibers can have a length of at least 20 microns and no greater than 500 microns. The slurry composition can include any distribution of fibers, e.g., bimodal, trimodal, etc.

The fibers of the composition can also have any suitable aspect ratio. In one or more embodiments, one or more fibers of the composition can have an average aspect ratio of at least 20 and no greater than 200.

The composition can include a homogeneous distribution of fibers. In one or more embodiments, the composition can include one or more fibers that are different in composition, dimension, etc., than one or more additional fibers in the composition.

In one or more embodiments, the slurry composition can be provided as a dry composition that includes the refractory material and the thixotropic agent, e.g., fibrillated fibers. In such embodiments, the dry composition can be shipped more easily. At least one of a binder and a solvent and any other desired compositions or components can be added to the dry composition to provide the final slurry composition.

In one or more embodiments, the slurry composition can also include a filler. Any suitable filler may be utilized. In one or more embodiments, the filler can include one or more bubbles. Any suitable bubbles can be utilized, e.g., glass bubbles. Suitable glass bubbles include 3M™ Glass Bubbles (available from 3M Company, St. Paul, Minn.). The glass bubbles can have any suitable density, e.g., at least 0.12 g/cc and no greater than 1.2 g/cc. Further, the glass bubbles can be present in any suitable amount in the slurry composition. In one or more embodiments, the glass bubbles can be present in the slurry composition in an amount of at least 0.25 weight percent and no greater than 5 weight percent, based on the overall weight of the composition.

Further, the glass bubbles can have any suitable dimensions. In one or more embodiments, the glass bubbles can have a particle size distribution of at least 60 μm and no greater than 120 μm in the effective top 95^(th) percentile of size distribution. The glass bubbles can have a single size distribution or two or more size distributions, e.g., a bimodal distribution.

The glass bubbles can include any suitable material or combination of materials. Further, in one or more embodiments, the glass bubbles can be hollow. The glass bubbles can also be disposed in any suitable matrix.

In one or more embodiments, the thixotropic agent can also include a polymer emulsion. In one or more embodiments, the polymer emulsion is an acrylic polymer emulsion. In one or more embodiments, the polymer emulsion is an acrylic polymer emulsion in water.

Polymers suitable for this application may be prepared using any of a number of different synthetic routes. Alkali-swellable polymers, for example, are synthesized by copolymerizing different monomers, where at least one monomer contains a carboxyl (—COOH) functional group. These polymers may have a structure that is linear, branched, or crosslinked to form a networked structure. Use of these polymers as thickening agents is described, for example, in U.S. Pat. No. 4,226,754 (Whitton et al), which discloses a polymer made by reacting an ester of methacrylic acid, methacrylic acid, and a vinyl ester of a saturated aliphatic carboxylic acid. These thickeners are often referred to as alkali-swellable thickeners because the carboxylic acid groups are sufficient to render the polymer water-soluble when neutralized with a suitable base.

In one or more embodiments, the refractory slurry includes hydrophobic entities covalently bonded to the polymeric backbone. For example, polymers can be formed by reacting an ethylenically unsaturated carboxylic acid monomer, a nonionic vinyl monomer, and a vinyl surfactant ester such as an alkylphenoxypoly (ethyleneoxy) ethyl acrylate terminated on one end with an alkyl phenyl group. Another example derives from a reaction product of an unsaturated carboxylic acid, alkyl (meth)acrylate, and an ester containing an alkyl phenyl group, where the alkyl group has 8 to 20 carbon atoms. These water-soluble polymers modified with hydrophobic moieties are described in U.S. Pat. No. 4,384,096 (Sonnabend) and U.S. Pat. No. 4,138,381 (Chang et al).

In one or more embodiments, the refractory slurry includes an acrylic emulsion copolymer is prepared using emulsion copolymerization of monomers falling within three of four classes of monomers, namely (meth)acrylic acid, alkyl (meth)acrylate, an ethoxylated ester of (meth)acrylic acid having a hydrophobic group and, optionally, a polyethylenically unsaturated monomer. In still other embodiments, the refractory slurry includes an emulsion copolymer based on the reaction product of monomers including methacrylic acid, ethyl acrylate, optionally a defined copolymerizable ethylenically unsaturated monomer, and a small weight percent of a polyethylenically unsaturated monomer. Advantageously, a wide range of surfactants can enhance the thickening effect on the composition when added to an aqueous system containing the copolymer when the emulsion copolymer is neutralized. The aforementioned copolymers are further described in European Patent No. 13,836 (Chang et al.) and U.S. Pat. No. 4,421,902 (Chang et al.).

In one or more embodiments, an alkali-swellable copolymer is synthesized as the reaction product of an ethylenically unsaturated carboxylic acid, a surface-active unsaturated ester, methacrylic acid esters or acrylic acid esters of aliphatic alcohols, and optionally one or more other ethylenically unsaturated comonomers, polyethylenically unsaturated compounds, and molecular weight regulators. The surface-active ester is terminated at one end with an aliphatic radical, which may be linear or branched, a mono-, di- or tri-alkyl phenyl radical with alkyl groups of 4 to 12 carbon atoms, or a block-copolymeric radical. On partial or complete neutralization, the copolymer becomes water-soluble or colloidally dispersible, and can be used as a thickening agent. These copolymers are also described in U.S. Pat. No. 4,668,410 (Engel et al.).

One particularly advantageous thixotropic agent usable in the refractory slurry 242 is a polymer emulsion based on hydrophobically modified ester of methacrylic acid available from Elementis Specialties in Hightstown, N.J. under the tradename RHEOLATE. Methods of making such polymer emulsions are described in detail, for example, in U.S. Pat. No. 6,069,217 (Nae et al.).

Another advantageous thixotropic agent, available from the same source and under the same tradename, is based on an aqueous hydrophobically modified alkali-soluble emulsion derived from an acrylic polymer and having about 30% solids by weight. Typically, this acrylic emulsion has a pH value of less than about 5.

The polymer emulsion can be present in an amount that increases the yield stress of the refractory slurry to an extent that enables use of only a single backup layer while preserving strength in the investment casting mold. In one or more embodiments, the polymer emulsion is present in an amount of at least 0.02 weight percent, at least 0.03 weight percent, at least 0.05 weight percent, at least 0.06 weight percent, or at least 0.07 weight percent, based on the overall weight of the composition. In one or more embodiments, the polymer emulsion is present in an amount of at most 1 weight percent, at most 0.9 weight percent, at most 0.8 weight percent, at most 0.75 weight percent, or at most 0.7 weight percent, based on the overall weight of the composition.

Advantageously, using a polymer emulsion as a thixotropic agent allows the refractory slurry to be operated within a shear stress regime that is much lower than that of the prior art while achieving a similar working viscosity for investment casting. In one or more embodiments, the refractory slurry displays a working viscosity of about 20 poise when subjected to a shear stress of at least 1 dyne per square centimeter, of at least 5 dynes per square centimeter, of at least 10 dynes per square centimeter, of at least 20 dynes per square centimeter, of at least 50 dynes per square centimeter, at least 100 dynes per square centimeter, at least 200 dynes per square centimeter, or at least 400 dynes per square centimeter, as measured using the method described in the Examples.

In one or more embodiments, the same slurry composition displays a working viscosity of about 20 poise when subjected to a yield stress in shear of at most 1000 dynes per square centimeter, at most 950 dynes per square centimeter, at most 900 dynes per square centimeter, at most 850 dynes per square centimeter, or at most 800 dynes per square centimeter.

Investment casting shells generally have large porosity as a result of the stuccoing process, which can adversely affect strength. For the strength to be deemed adequate for a given application, it must be capable of withstanding potentially high internal pressure and thermal stress, especially during the de-waxing process and when pouring metal into the free standing ceramic shell. Cracking can occur when the stress on the mold is greater than the modulus of rupture of the mold material. In some embodiments, the investment casting mold has non-fired modulus of rupture of at least about 1 MPa, or at least 1.75 MPa, after being fully hardened. In some embodiments, the investment casting mold has non-fired modulus of rupture of at most 5 MPa after being fully hardened.

In one or more embodiments, the slurry composition further includes an aluminum phyllosilicate clay. In some embodiments, the aluminum phyllosilicate clay is present in an amount ranging from a weight ratio of at least 1:15, at least 1:10, at least 1:8, at least 1:7, or at least 1:6, relative to that of the polymer emulsion. In some embodiments, the aluminum phyllosilicate clay is present in an amount ranging from a weight ratio of at most 6:1, at most 5:1, or at most 4:1, relative to that of the polymer emulsion.

Combining a thixotropic thickener that includes a polymer emulsion, particularly an acrylic emulsion, with an aluminum phyllosilicate clay may provide certain synergistic effects in the investment mold. For example, inclusion of both the polymer emulsion thickener and the aluminum phyllosilicate clay in the backup refractory slurry may substantially increase the working time of the slurry as compared with including only the aluminum phyllosilicate as thickener. When the aluminum phyllosilicate clay is used on its own, the backup slurry may continue to drain off of the pattern. Moreover, inclusion of both the polymer emulsion and the aluminum phyllosilicate clay may be preferred over inclusion of the polymer emulsion alone as the latter may produce slurries that can be too viscous. Such high viscosities in turn can cause delicate patterns to crack or break when inserted into the slurry. In sum, the combination of a polymer emulsion thickener and an aluminum phyllosilicate clay may provide an unexpected and advantageous balance of flowability along with a long working time.

There are no particular restrictions on the overall solids present in the refractory slurry, but this measure should fall within a range sufficient to enable a stable colloidal suspension and yield a robust final investment casting mold 100. In one or more embodiments, the slurry composition has an overall solids content of at least 45 weight percent, at least 50 weight percent, or at least 55 weight percent, based on the overall weight of the composition. In one or more embodiments, the slurry composition has an overall solids content of at most 85 weight percent, at most 80 weight percent, or at most 75 weight percent, based on the overall weight of the composition.

FIG. 3 is a schematic cross-section view of another embodiment of an investment casting mold 200. All of the design considerations and possibilities regarding the mold 100 of FIGS. 1-2 apply equally to the mold 200 of FIG. 3. The mold 200 shares some characteristics of the mold 100. For example, like mold 100, the mold 200 includes a prime layer 204 disposed on a wax pattern 202 and an intermediate layer 210 disposed on the prime layer 204. The pattern 202, prime layer 204, and intermediate layer 210 generally share the aforementioned features, options, and advantages described regarding the mold 100. Here, the prime layer 204 includes an inner coating of zircon-containing slurry 206 followed by an outer layer of zircon stucco 208. The intermediate layer 210, in the illustrated embodiment, includes an inner coating of refractory slurry 212 and an outer layer of refractory stucco 214. The intermediate slurry layer 210 may also contain a zircon refractory.

A single backup layer 240 is disposed on the intermediate layer 210. As shown, the backup layer 240 has a spatial thickness considerably greater than either of the prime or intermediate layers 204, 210. Advantageously, and as shown, the backup layer 240 can fill in open undercuts and cavities presented by the branches of the pattern 202, thereby simplifying subsequent coating processes. As a further major benefit, the configuration of the mold 200 eliminates the need for multiple backup layers in common investment casting applications. The backup layer 240, as shown, includes an inner coating of a refractory slurry 242 followed by a layer of refractory stucco 244. Finally, a seal layer 234 is disposed over the backup layer 240, whereby the two layers 234, 240 directly contact each other. The seal layer 234, which serves the same purposes as those of the seal layer 134, can also be omitted if desired.

In the above method, each slurry layer is optionally disposed on the pattern or underlying layer using a dipping process. When a dipping process is used, it is advantageous for the slurry to have a sufficient viscosity to be retained on the pattern or underlying layer over an acceptable working time, yet also having sufficient flowability to fill essentially all of the voids in the dipped assembly to preserve high fidelity in the mold shape. Acceptable working times generally range from about 12 seconds to about 60 seconds. The required working time for this slurry will depend upon the process and foundry but generally is the time required for the slurry to stop draining and then be moved from above the slurry pot into the stucco application area. Using suitable techniques, this time period is on the order of 2-3 minutes. These competing properties can be simultaneously achieved using the investment casting molds and methods described herein.

In one or more embodiments, the investment casting mold 200 is fabricated using methods of layer-by-layer construction analogous to those used to fabricate the investment casting mold 100, but with certain deviations as noted herein. Generally, departures from known techniques include differences in the composition of the refractory slurry used for the backup layer(s) and, advantageously, reduction in the number of processing steps required to produce the finished investment casting mold 200.

Alternative embodiments are shown in FIGS. 4 and 5. FIG. 4 depicts an investment casting mold 300 according to another embodiment in which an outermost seal layer is omitted. All of the design considerations and possibilities regarding the mold 100 of FIGS. 1-2 apply equally to the mold 300 of FIG. 4. This three-layered construction includes a prime layer 304 extending across and contacting a sacrificial pattern 302, an intermediate layer 310 extending across and contacting the prime layer 304, and a single backup layer 340 extending across and contacting the intermediate layer 310. Like in the embodiment previously described, each of the layers 304, 310, 340 includes an inner sub-layer of refractory slurry adjoining an outer sub-layer of a refractory stucco.

Absent from the mold 300 is an outermost seal layer; in FIG. 4, the layered construction ends with the refractory stucco for the backup layer 340. While sharing most of the functional properties of the mold 200, the mold 300 requires even fewer processing steps to fabricate.

FIG. 5 illustrates an investment casting mold 400 according to yet another embodiment. All of the design considerations and possibilities regarding the mold 100 of FIGS. 1-2 apply equally to the mold 400 of FIG. 5. Compared with prior embodiments, the mold 400 is notably even further simplified in its two-layered construction. Showing merely a prime layer 404 and backup layer 440 disposed on a pattern 402, the mold 400 can advantageously be made using only two dips—one for each of layers 404, 440. Other aspects of the mold 400 and its constituent layers are essentially the same as those described with respect to the three- and four-layered embodiments above.

Ideally, an investment casting refractory slurry displays a yield stress that is sufficient to prevent excessive drainage of the slurry from a pattern after the pattern is withdrawn from a bath of the slurry. This characteristic should be tempered, however, by its flowability—essentially, its ability to flow into and around complex pattern geometries, including narrow cavities, when the pattern is dipped into the slurry. The refractory slurries provided here operate in a solid-like regime at the low shear rates associated with gravity, but operate in a liquid-like regime at higher shear rates associated with dipping the pattern into a bath of the slurry. By minimizing gravity-induced drainage while simultaneously achieving good flowability in the dipping process, the provided compositions reduce the number of required dips while preserving the fidelity of the final molded product.

In some embodiments, the yield stress of the slurry composition is at least 0.2 dynes/cm², at least 0.5 dynes/cm², at least 1 dyne/cm², at least 5 dynes/cm², at least 10 dynes/cm², or at least 50 dynes/cm². In the same or alternative embodiments, the yield stress of the refractory slurry can be at most 100 dynes/cm², 200 dynes/cm², at most 250 dynes/cm², at most 500 dynes/cm², at most 750 dynes/cm², or at most 1000 dynes/cm². Exemplary refractory slurries or compositions, at the onset of flow, can display a viscosity at the onset of flow of at least 20 cP and no greater than 40,000 cP.

Any suitable technique or combination of techniques can be utilized to form investment casting mold that includes the refractory slurries described herein. In one or more embodiments, a sacrificial pattern can be coated with a prime layer that includes a first refractory slurry and a first refractory stucco. The prime layer can be at least partially hardened using any suitable technique or combination of techniques. The prime layer can be coated with an intermediate layer that includes a second refractory slurry and a second refractory stucco. Such intermediate layer can be at least partially hardened using any suitable technique or combination of techniques. The intermediate layer can be coated by a backup layer that includes a thixotropic agent that includes, e.g., fibrillated fibers. The backup layer can be at least partially hardened using any suitable technique or combination of techniques. In one or more embodiments, one or more glass bubbles can be included in the backup layer.

As mentioned herein, one or more embodiments of refractory slurries can be provided as dry compositions and then mixed with one or more solvents or liquids using any suitable technique or combination of techniques. For example, a dry composition that includes a refractory material, glass bubbles, and a thixotropic agent (e.g., fibrillated fibers) can be provided to form a dry composition using any suitable technique or combination of techniques. The dry composition can be combined with at least one of a binder and a solvent to form a refractory slurry using any suitable technique or combination of techniques.

EXAMPLES

Materials

“WDS II”, fused silica flour was obtained from 3M Midway, Midway, Tenn., under the trade designation “WDS II”.

“WDS 3”, fused silica flour, was obtained from 3M Midway, Midway, Tenn., under trade designation “WDS 3”.

“MIN-SIL 120F”, fused silica flour, was obtained from 3M Midway, Midway, Tenn., under trade designation “MIN-SIL 120F”.

“NALCO 1130”, silica sol, 30 weight % SiO₂, 8 nanometer particle size, was obtained from Nalco Chemical Company, Naperville, Ill., under trade designation “NALCO 1130”.

“NALCO 1030”, silica sol, 30 weight % SiO₂, 11-16 nm particle size, was obtained from Nalco Chemical Company, Naperville, Ill., under trade designation “NALCO 1030”.

“MINCO HP”, a styrene butadiene latex polymer, 50 weight % solids, was obtained from 3M Midway, Midway, Tenn., under trade designation “Minco HP”.

“NALCO 2305”, antifoam additive containing a blend of silicones and polyglycols in a hydrocarbon solvent, was obtained from Nalco Chemical Company, Naperville, Ill., under trade designation “NALCO 2305”.

“NALCO 8815”, a wetting agent, was obtained from Nalco Chemical Company, Naperville, Ill., under trade designation “NALCO 8815”.

“BENTONE EW”, highly beneficiated, easily dispersible powdered clay thickener, was obtained from Elementis, Specialties, Inc., Hightstown, N.J., under trade designation “BENTONE EW”.

“RHEOLATE 420”, an alkali swellable thickener, was obtained from Elementis, Specialties, Inc., Hightstown, N.J., under trade designation “RHEOLATE 420”.

Fibrillated fibers, a high-density polyethylene (HDPE) fibrillated fibers 0.1 mm length and 5 micrometers diameter, obtained from Minifibers, Inc., Johnson City, Tenn. under trade designation “SHORT STUFF FIBRILLATED HDPE”.

Fused silica, 50×100 mesh (finer than U.S. Sieve 50 mesh but coarser than U.S. Sieve 100 mesh), was obtained from 3M Midway, Midway, Tenn.

Fused silica, 30×50 mesh (finer than U.S. Sieve 30 mesh but coarser than U.S. Sieve 50 mesh), was obtained from 3M Midway, Midway, Tenn.

“HGS4K28”, glass bubbles, obtained from 3M Company, St. Paul, Minn. under trade designation “HGS4K28”.

General Method for Preparing Prime Slurry, Intermediate Slurry, and Backup Slurry

De-ionized (DI) water and NALCO 1130 silica sol were added into a sufficient volume container. While mixing using a INDCO Model HS120T mixer (2 horsepower, 220 V, single phase motor, set at a speed of 2050 rpm), desired amounts of silica flour, additives such as polymeric binders (e.g., styrene-butadiene latex), and antifoam and/or wetting agents were added and mixing was continued until all the lumps were dispersed. Finally, if desired, a rheological additive (i.e., a thixotropic agent) was added and mixing was continued, typically for less than 5 minutes but up to as long as 30 minutes.

General Method for Preparing Investment Casting Molds

Investment casting molds were made using a multi-step process. First, a wax pattern having the shape of final investment cast parts was provided. On top of the wax pattern, investment cast molds were formed by building a series of shells (i.e., layers) sequentially. In a first step, the wax pattern was coated by a “prime layer” that included an initial coating of prime slurry layer that was further coated with a prime stucco layer. The prime slurry layer was formed by dipping the wax pattern in the prime slurry for about 20 seconds while rotating and moving the wax pattern to maximize the uniformity of the prime slurry layer. A prime stucco layer was then deposited on the wet prime slurry layer by exposing the wax pattern with the prime slurry layer thereon to a fluidized bed of zircon particles. The wax pattern with the prime stucco layer was then dried at 21 degrees Celsius for about 2 hours. Afterwards, the wax pattern with the dried prime layer was coated with an “intermediate layer” in essentially the same manner as the prime layer except by using an intermediate slurry and stucco layers that was then dried. The intermediate stucco layer was formed using a fluidized bed of 50×100 mesh fused silica particles. The composition of the intermediate slurry could be the same as or different from the primary slurry. The resulting pattern was then coated with one or more backup layer(s) in essentially the same manner as the primary/intermediate layer except backup slurry and stucco layers were used and then dried. The backup stucco layer was formed using a fluidized bed of 30×50 mesh fused silica particles. The composition of the backup slurry could be the same as or different from the primary/intermediate slurry. The backup slurry layer/stucco layer building is typically repeated several times to build enough thickness with sufficient drying between each layer. Finally, the pattern with sufficiently thick backup layer(s) was coated with a seal layer by dipping it again into the backup slurry and drying. The final investment casting molds were freed of the wax pattern, fired, and used for testing and/or for preparing final investment cast parts.

Example and Comparative Example investment casting molds, prepared as above, were characterized in their “green” states and/or after firing.

Method for Measuring Viscosity

Viscosity and shear stress data for slurries were measured using an AR G2 stress controlled rheometer (TA Instruments, New Castle, Del.) outfitted with a 40-mm diameter parallel plate fixture. Measurements were made using a gap of 1 mm and an operating temperature of 23 degrees Celsius.

Slurries were tested using a continuous flow shear rate sweep. Tests were conducted with an ascending shear rate from 10⁻³ s⁻¹ to 100 s⁻¹, and then descending shear rate down to 10⁻³ s⁻¹. The yield stress of each slurry was obtained by plotting shear stress as a function of total strain for ascending shear rates, identifying regimes of fluid-like and solid-like behavior along the plot, fitting a power law to each regime, then determining shear stress at the intersection point between these fits. The viscosity at the onset of flow was also determined based on the measured viscosity at the time yield stress was first reached.

Method for Determining the Bend Strength

To prepare strength testing samples, standard stainless steel bars 1 in.×0.25 in.×13 in. (2.54 cm×0.64 cm×33 cm) were covered with investment casting shells prepared from slurries used in the Examples and Comparative Examples in the same manner as preparing the investment cast molds described above. Before coating with the investment casting shells, the steel bars were first coated with wax (S.C. Johnson's Paste Wax, commercially available from S.C. Johnson & Sons, Inc., Racine, Wis.). The resulting shells were separated from the steel plates and were used for bend strength testing. The strength testing of the shell samples were carried out using a Universal Test Machine (Model SS™-1, obtained from United Test Machine of Huntington Beach, Calif.) using a cross head speed of 0.05 in. (0.13 cm) per minute along with a 2 in. (5 cm) span. The thickness of the test samples at break was measured in six locations across the break, three on either side of the break and the measurements averaged. The width was measured twice and the measurements averaged. The strength test data reported was average of 24 test samples for each Example and Comparative Example investment cast mold composition. The strength data for the Examples were run along with the corresponding Comparative Examples samples. The strength test data e.g., modulus of rupture (MOR), modulus of elasticity (MOE), and load at failure were determined. The strength testing was done in the green and fired states under a variety of environmental conditions.

Method for Permeability and Burst Testing

For this test, samples were prepared by building shells using the slurries prepared according to the Examples and Comparative Examples, on polyvinylchloride (PVC), schedule 40 cold plumbing pipes. The PVC pipes had 0.75 in. (1.09 cm) inner diameter and 1.05 in. (2.77 cm) outer diameter and were 13 in. (33 cm) long. The pipes were first coated with wax (S.C. Johnson's Paste Wax). After the shells were built, the resulting samples were cut into 6 in. (15.2 cm) long sections for testing. The permeability and burst testing was done using the method described in Snyder, B. and Snow, J. “A New Combination Shell Strength and Permeability Test,” in the 51^(st) Annual Technical Meeting of the Investment Casting Institute, 2003, p. 11:1-25 (published by the Investment Casting Institute). Ten sections (i.e., samples) were tested for each Example and Comparative Example.

Method for Testing Slurry Draining and Shell Building

Slurry draining/shell building comparisons were made using 4″×4″×0.25″ (10.16 cm×10.16 cm×0.63 cm) wax plates. The wax plates were dipped, without rotation, in the slurry to be tested. Then, the wax plates were removed from the slurry and the weight of the slurry retained on the wax plate was monitored for 1 minute (by weighing at 2 second intervals). The change in the weight of retained slurry was plotted versus time. The weight of the slurry retained on the wax plate at the end of 1 minute was recorded. Then the slurry-coated wax plate was stucco coated using 50×100 mesh fused silica particles and the weight of the retained stucco particles was recorded. Finally, the total weight retained on the wax plate (slurry+stucco) was calculated.

The above process was repeated for five times, recording the weight of slurry, stucco, and the total (slurry+stucco) weight after each dipping.

Comparative Example A and B (CE-A and CE-B)

CE-A investment cast mold was prepared using the general method for preparing investment casting molds described above. First, the prime layer was applied by dipping the wax pattern in a prime slurry. The composition of the prime layer (prepared as described above to 76 wt. % solids slurry) was as follows:7000 g zircon flour (200F mesh), 7000 g WDS II fused silica flour (200F mesh), 400 g NALCO 1030 silica sol, 350 g MINCO HP latex, 10 g NALCO 8815 wetting agent, and 2 g NALCO 2305 antifoam additive. The stucco layer (50×100 mesh zircon particles) was applied to the wet prime layer and the pattern was dried for 2 hours at 21° C. Then the resulting mold assembly was coated with an intermediate layer and stucco. The composition of the intermediate slurry (prepared as described above) was as follows 13,705 g of WDS II silica flour, 4,516 g of NALCO 1030, 934 g DI water, 498 g MINCO HP latex binder, and 21 g NALCO 2305 antifoam additive. The intermediate stucco was applied using 50×100 mesh fused silica particles. After drying the mold assembly at 21° C. for 2 hours, one backup layer was applied. The composition of the backup slurry (prepared as described above to 62.7 wt. % solids slurry) was as follows: 387 g MIN-SIL 120F silica flour, 220 g of NALCO 1030, 9.4 g MINCO HP latex binder, 1.71 g RHEOLATE 420 and 1.3 g BENTONE EW. The backup stucco was applied using 30×50 mesh fused silica particles. After drying the mold assembly at 21° C. for 2 hours. Finally, a seal layer was applied and the resulting mold assembly was dried at 21° C. for 2 hours. The seal layer had the same composition as the intermediate slurry.

CE-B investment cast mold was prepared in the same manner as CE-A, except that NALCO 1030 silica sol was replaced with NALCO 1130 silica sol.

The CE-A and CE-B investment cast molds were fired at 2000° F. (1093° C.) for two hours before use.

Examples 1 and 2 (EX-1 and EX-2)

EX-1 investment cast mold was prepared in the same manner as CE-A, except that the backup refractory slurry (prepared as described above to 62.1 wt. % solids) was as follows: 2480 g MIN-SIL 120F silica flour, 1450 g of NALCO 1030, 100 g MINCO HP latex binder, 50 g HGS4K28 glass bubbles, and 11 g BENTONE EW and 1 g fibrillated fibers.

EX-2 investment cast mold was prepared in the same manner as EX-1, except that NALCO 1030 silica sol was replaced with NALCO 1130 silica sol.

The EX-1 and EX-2 investment cast molds were fired at 2000° F. (1093° C.) for two hours before use.

FIGS. 6a and 6b show optical micrographs of CE-B and EX-2 investment casting molds, respectively.

The CE-A, CE-B, EX-1 and EX-2 formulations were used to prepare sufficient number of permeability, burst, and strength test samples for testing under a variety of test conditions as described below. Sample preparation and testing was carried out using the procedures described above. Test results obtained are described below.

Shell Permeability and Burst Strength

CE-A, CE-B, EX-1, and EX-2 samples prepared for strength testing and permeability testing were used to determine the permeability and burst strength of the shells built for each formulation. Table 1, below, summarizes the permeability and the maximum tangential hoop stress for CE-A, CE-B, EX-1, and EX-2 obtained using the methods described above.

TABLE 1 Max. Tangential Hoop Stress Test Strength Test Samples Maximum 2X 95% Permeability 95% Standard tangential Standard Example (cm²) error (cm²) stress (MPa) error (MPa) CE-A 2.4 × 10⁻¹⁰ 1.4 × 10⁻¹¹ 0.23 0.012 CE-B 4.7 × 10⁻¹⁰ 4.4 × 10⁻¹¹ 0.15 0.017 EX-1 2.5 × 10⁻¹⁰ 3.0 × 10⁻¹¹ 0.21 0.009 EX-2 5.8 × 10⁻¹⁰ 9.0 × 10⁻¹² 0.19 0.012

Shell Thickness

The shells were invested on flat 1″×14″ (2.5 cm×36 cm) stainless steel bars (MOR sample shells) and ¾″ (1.9 cm) PVC pipes.

MOR sample shell thickness was recorded and is shown in Table 2.

TABLE 2 Example Shell Thickness (cm) 95% Standard Error (cm) CE-A 0.57 0.023 CE-B 0.61 0.023 EX-1 0.61 0.028 EX-2 0.69 0.031

Pipe shell thicknesses are depicted in Table 3 below.

TABLE 3 Example Shell Thickness (cm) 2 × 95% Standard Error (cm) CE-A 0.53 0.00 CE-B 0.58 0.03 EX-1 0.61 0.15 EX-2 0.66 0.03

Green Shell Properties

Green shell samples were tested in the unfired/green state using the method described above. The results of the test are summarized below in Table 4.

TABLE 4 MOR/ MOE/ Failure Load/ 95% Standard 95% Standard 95% Standard Example Error (MPa) Error (MPa) Error (kg) CE-A 3.67/0.19 1889/199  15.4/1.09 CE-B 4.33/0.18 1744/186  10.0/0.82 EX-1 4.44/0.19 1592/234 13.61/0.59 EX-2 3.17/0.14 1551/228 12.25/0.77

Hot/Wet Shell Testing

Green shell samples were tested “Hot/Wet” after boiling for 15 minutes using the method described above. Table 5, below, illustrates the MOR, MOE, and failure load for the molds under hot/wet testing.

TABLE 5 MOR/ MOE/ Failure Load/ 95% Standard 95% Standard 95% Standard Example error (MPa) error (MPa) error (kg) CE-A 1.08/0.14 820/97 1.0/0.13 CE-B 1.15/0.08  710/124 1.5/0.13 EX-1 0.97/0.10 476/62 1.1/0.18 EX-2 0.99/0.10 551/83 1.6/0.13

Fired Cold Shell Properties

Knockout-type properties were estimated after firing MOR bar shell samples to 2000° F. (1093° C.), holding for 2 hours, and allowing to furnace cool.

Table 6 below summarizes the fired (after cooling to room temperature) strength test data for CE-A, CE-B, EX-1, and EX-2 obtained using the method described above.

TABLE 6 MOR/ MOE/ Failure Load/ 95% Standard 95% Standard 95% Standard Example Error (MPa) Error (MPa) Error (kg) CE-A 2.08/0.10 1124/103 1.81/0.18 CE-B 1.59/0.13 586/97 2.04/0.14 EX-1 1.92/0.17 855/97 2.22/0.27 EX-2 1.31/0.15  717/110 2.00/0.24

Fired-Hot Shell Properties

Table 7 summarizes the fired (and tested while hot) strength test data for CE-A, CE-B, and EX-1 and EX-2, prepared using the method described above. Shell strength was tested after firing the molds at 2000° F. (1093° C.) for two hours and broken while still hot.

TABLE 7 MOR/ MOE/ Failure Load/ 95% Standard 95% Standard 95% Standard Example Error (MPa) Error (MPa) Error (kg) CE-A 8.56/0.89 4612/683 7.71/0.82 CE-B 7.39/0.60 3709/434 8.62/0.68 EX-1 8.35/0.68 4040/669 10.43/0.68  EX-2 5.90/0.52 2827/559 8.17/0.64

Viscosities

The viscosity of the CE-B and EX-2 backup slurries was measured using the test method described above. FIG. 7 is a graph of the shear stress versus shear rate for CE-B and EX-2 backup slurries. The viscosity at the onset of flow for CE-B and EX-2 backup slurries was also determined based on the measured viscosity at the time yield stress was first reached. The viscosity at the onset of flow for the CE-B and EX-2 backup slurries were determined to be 11,812 cP and 18,605 cP, respectively.

Comparative Example C (CE-C) and Examples 3 and 4 (EX-3 and EX-4)

CE-C investment cast mold was prepared using the general method for preparing investment casting molds described above. Each of prime, intermediate, backup, and seal layers was applied once. The composition of the prime, intermediate, backup, and seal slurries (prepared as described above to 64.9 wt. % solids) was the same as follows: 13300 g WDS3 fused silica flour, 6300 g NALCO 1030 silica sol, 600 g MINCO HP latex, 12 g NALCO 8815 wetting agent, 12 g NALCO 2305 antifoam additive and 285 g deionized (DI) water. After the application of the seal layer, the resulting mold assembly was dried at 21° C. for 2 hours.

EX-3 investment cast sols were prepared in the same manner as CE-C except that the backup slurry used to form the backup layer had a different composition. Specifically, the backup slurry used to form the backup layer of EX-3 sample further included 27 g of short stuff fiber added to the composition.

EX-4 investment cast sols were prepared in the same manner as CE-C except that the backup slurry used to form the backup layer had a different composition. Specifically, the backup slurry used to form the backup layer of EX-3 sample further included 54 g of short stuff fiber added to the composition.

The CE-C, EX-3 and EX4 investment cast molds were fired at 2000° F. (1093° C.) for two hours before use.

FIGS. 8a, 8b, and 8c show optical micrographs of above prepared CE-C, EX-3, and EX-4 investment casting molds, respectively.

The CE-C, EX-3, and EX-4 formulations were used to prepare sufficient numbers of permeability, burst, and strength test samples for testing under a variety of test conditions as described below. Sample preparation and testing was carried out using the procedures described above. Test results obtained are described below.

Slurry Draining and Shell Building

Slurry draining/shell building comparisons were made as described above using the CE-C, EX-3, and EX-4 backup slurries.

FIGS. 9A and 9B show the change in weight of slurry retained on a wax plate after first (FIG. 9A) and third dip (FIG. 9B) for CE-C, EX-3 and EX-4 backup slurries (data obtained at 2-second intervals for a total of 60 seconds).

FIGS. 10A, 10B, and 10C show the weight of slurry (after 1 minute of draining), stucco, and total weight (slurry+stucco) retained on wax plates after first dip (FIG. 10A), third dip (FIG. 10B) and total after five dips (FIG. 10C) for CE-C, EX-3 and EX-4 backup slurries.

The addition of fibrillated fibers to backup slurries increased the weight of the slurry retained on the wax plates.

Shell Permeability and Burst Strength

CE-C, EX-3, and EX-4 samples prepared for strength testing and permeability testing were used to determine the permeability and burst strength of the shells built for each formulation. Table 8, below, summarizes the permeability and the maximum tangential hoop stress for CE-C, EX-3, and EX-4 obtained using the methods described above.

TABLE 8 Max. Tangential Hoop Stress Test Strength test samples Maximum 2X 95% Permeability 95% Standard Tangential Standard Example (cm²) Error (cm²) Stress (MPa) Error (MPa) CE-C 1.0 × 10⁻⁹ 2.93 × 10⁻¹⁰ 0.22 0.021 EX-3 1.0 × 10⁻⁹ 1.59 × 10⁻¹⁰ 0.23 0.021 EX-4 1.0 × 10⁻⁹ 1.58 × 10⁻¹⁰ 0.13 0.021

Shell Thickness

The shells were invested on flat 1″×14″ (2.5 cm×36 cm) stainless steel bars (MOR sample shells) and ¾″ (1.9 cm) PVC pipes.

MOR sample shell thickness was recorded and is shown in Table 9.

TABLE 9 Example Shell Thickness (cm) 95% Standard Error (cm) CE-C 0.84 0.025 EX-3 0.86 0.025 EX-4 1.02 0.025

Pipe shell thicknesses are depicted in Table 10 below.

TABLE 10 Example Shell Thickness (cm) 2 × 95% Standard Error (cm) CE-C 0.84 0.025 EX-3 0.86 0.025 EX-4 0.99 0.025

Green Shell Properties

Green shell samples were tested in the unfired/green state using the method described above. The results of the test are summarized below in Table 11.

TABLE 11 MOR/ MOE/ Failure Load/ 95% Standard 95% Standard 95% Standard Example Error (MPa) Error (MPa) Error (kg) CE-C 2.38/0.17 758/110 5.58/0.32 EX-3 2.81/0.15 869/124 6.94/0.45 EX-4 2.55/0.16 524/83  9.30/0.64

Hot/Wet Shell Testing

Green shell samples were tested “Hot/Wet” after boiling for 15 minutes using the method described above. Table 12, below, illustrates the MOR, MOE, and failure load for the molds under hot/wet testing.

TABLE 12 MOR/ MOE/ Failure Load/ 95% Standard 95% Standard 95% Standard Example Error (MPa) Error (MPa) Error (kg) CE-C 0.88/0.10 414/69 2.00/0.18 EX-3 0.88/0.08 400/62 2.27/0.18 EX-4 0.97/0.12 297/83 3.31/0.26

Fired Cold Shell Properties

Knockout-type properties were estimated after firing MOR bar shell samples to 2000° F. (1093° C.), holding for 2 hours and allowing to furnace cool.

Table 13 below summarizes the fired (after cooling to room temperature) strength test data for CE-C, EX-3, and EX-4 obtained using the method described above.

TABLE 13 MOR/ MOE/ Failure Load/ 95% Standard 95% Standard 95% Standard Example Error (MPa) Error (MPa) Error (kg) CE-C 1.31/0.08 483/69 3.13/0.18 EX-3 1.41/0.09 345/48 3.31/0.26 EX-4 1.29/0.10 269/35 3.99/0.45

Fired-Hot Shell Properties

Table 14 summarizes the fired (and tested while hot) strength test data for CE-C, and EX-3 and EX-4, prepared using the method described above. Shell strength was tested after firing the molds at 2000° F. (1093° C.) for two hours and broken while still hot.

TABLE 14 MOR/ MOE/ Failure Load/ 95% Standard 95% Standard 95% Standard Example Error (MPa) Error (MPa) Error (kg) CE-C 7.53/0.53 2102/234 17.69/0.91 EX-3 7.36/0.35 1531/228 19.05/1.41 EX-4 6.45/0.39 1027/152 22.23/1.50

REPRESENTATIVE EMBODIMENTS Embodiment 1

A slurry composition for investment casting comprising: a refractory material; a binder; a solvent; and a thixotropic agent comprising fibrillated fibers.

Embodiment 2

The composition of embodiment 1, wherein the fibrillated fibers comprise organic fibers.

Embodiment 3

The composition of Embodiment 2, wherein the organic fibers comprise high-density polyethylene.

Embodiment 4

The composition of any one of Embodiments 1-3, further comprising an aluminum phyllosilicate clay.

Embodiment 5

The composition of any one of Embodiments 1-4, wherein the fibrillated fibers are present in an amount of at least 0.005 weight percent and no greater than 1.5 weight percent, based on the overall weight of the composition.

Embodiment 6

The composition of any one of Embodiments 1-5, wherein the composition has an overall solids content ranging from 45 weight percent to 85 weight percent, based on the overall weight of the composition.

Embodiment 7

The composition of any one of Embodiments 1-6, wherein the binder comprises colloidal silica.

Embodiment 8

The composition of any one of Embodiments 1-7, wherein the composition comprises a working viscosity of 20 poise when subjected to a shear stress of at least 1 dyne per square centimeter and at most 1000 dynes per square centimeter.

Embodiment 9

The composition of any one of Embodiments 1-8, wherein the binder comprises a styrene-butadiene latex.

Embodiment 10

The composition of any one of Embodiments 1-8, wherein the binder comprises a polyvinyl butyral resin.

Embodiment 11

An investment casting mold made using the composition of any one of Embodiments 1-10.

Embodiment 12

A method of making an investment casting mold comprising: coating a sacrificial pattern with a prime layer comprising a first refractory slurry and a first refractory stucco; at least partially hardening the prime layer; coating the prime layer with an intermediate layer comprising a second refractory slurry and a second refractory stucco; at least partially hardening the intermediate layer; coating the intermediate layer with a backup layer comprising a thixotropic agent that comprises fibrillated fibers; and at least partially hardening the backup layer.

Embodiment 13

The method of Embodiment 12, wherein the investment casting mold has a modulus of rupture of at least 0.5 MPa and no greater than 10 MPa after being fully hardened.

Embodiment 14

A slurry composition for investment casting comprising: a refractory material;

-   -   glass bubbles; and a thixotropic agent comprising fibrillated         fibers.

Embodiment 15

The composition of Embodiment 14, further comprising a binder and a solvent.

Embodiment 16

The composition of Embodiment 15, wherein the composition comprises a viscosity at the onset of flow of at least 5,000 cP and no greater than 50,000 cP.

Embodiment 17

The composition of any one of Embodiments 14-16, wherein the fibrillated fibers comprise organic fibers.

Embodiment 18

The composition of Embodiment 17, wherein the organic fibrillated fibers comprise high-density polyethylene.

Embodiment 19

The composition of any one of Embodiments 14-18, wherein the glass bubbles comprise a density of at least 0.12 g/cc and no greater than 1.2 g/cc.

Embodiment 20

The composition of any one of Embodiments 14-19, wherein the glass bubbles are present in an amount of at least 0.1 weight percent and no greater than 10 weight percent, based on the overall weight of the composition.

Embodiment 21

The composition of any one of Embodiments 14-20, wherein the fibrillated fibers are present in an amount of at least 0.005 weight percent and no greater than 1.5 weight percent, based on the overall weight of the composition.

Embodiment 22

The composition of Embodiment 14, wherein the composition is a dry composition.

Embodiment 23

A method of forming a slurry composition for investment casting, comprising:

-   -   providing a dry composition comprising a refractory material,         glass bubbles, and a thixotropic agent comprising fibrillated         fibers; and combining the dry composition with at least one of a         binder and a solvent to form the slurry composition.

Embodiment 24

An investment casting mold comprising: a prime layer; an intermediate layer disposed on the prime layer; a backup layer disposed on the intermediate layer; and a seal layer disposed on the backup layer; wherein at least one of the prime layer, intermediate layer, backup layer, and seal layer comprises a slurry composition comprising a refractory material and a thixotropic agent comprising fibrillated fibers.

Embodiment 25

The mold of Embodiment 24, wherein the refractory composition further comprises glass bubbles.

All references and publications cited herein are expressly incorporated herein by reference in their entirety into this disclosure, except to the extent they may directly contradict this disclosure. Illustrative embodiments of this disclosure are discussed and reference has been made to possible variations within the scope of this disclosure. These and other variations and modifications in the disclosure will be apparent to those skilled in the art without departing from the scope of the disclosure, and it should be understood that this disclosure is not limited to the illustrative embodiments set forth herein. Accordingly, the disclosure is to be limited only by the claims provided below. 

1. A slurry composition for investment casting comprising: a refractory material; a binder; a solvent; glass bubbles; and a thixotropic agent comprising fibrillated fibers.
 2. The composition of claim 1, wherein the fibrillated fibers comprise organic fibers.
 3. The composition of claim 2, wherein the organic fibers comprise high-density polyethylene.
 4. The composition of claim 1, further comprising an aluminum phyllosilicate clay.
 5. The composition of claim 1, wherein the fibrillated fibers are present in an amount of at least 0.005 weight percent and no greater than 1.5 weight percent, based on the overall weight of the composition.
 6. The composition of claim 1, wherein the composition has an overall solids content ranging from 45 weight percent to 85 weight percent, based on the overall weight of the composition.
 7. The composition of claim 1, wherein the binder comprises colloidal silica.
 8. The composition of claim 1, wherein the composition comprises a working viscosity of 2 Pascal·seconds when subjected to a shear stress of at least 0.1 N/m² and at most 100 N/m², as measured according to the Method for Measuring Viscosity.
 9. The composition of claim 1, wherein the binder comprises a styrene-butadiene latex.
 10. The composition of claim 1, wherein the binder comprises a polyvinyl butyral resin.
 11. An investment casting mold made using the composition of claim
 1. 12. A method of making an investment casting mold comprising: coating a sacrificial pattern with a prime layer comprising a first refractory slurry and a first refractory stucco; at least partially hardening the prime layer; coating the prime layer with an intermediate layer comprising a second refractory slurry and a second refractory stucco; at least partially hardening the intermediate layer; coating the intermediate layer with a backup layer comprising the composition according to claim 1; and at least partially hardening the backup layer.
 13. The method of claim 12, wherein the investment casting mold has a modulus of rupture of at least 0.5 MPa and no greater than 10 MPa after being fully hardened.
 14. A composition for investment casting comprising: a refractory material; glass bubbles; and a thixotropic agent comprising fibrillated fibers.
 15. The composition of claim 14, further comprising a binder and a solvent.
 16. The composition of claim 15, wherein the composition comprises a viscosity at the onset of flow of at least 5 Pascal·seconds and no greater than 50 Pascal·seconds, as measured according to the Method for Measuring Viscosity.
 17. The composition of claim 14, wherein the composition is a dry composition.
 18. A method of forming a slurry composition for investment casting, comprising: providing the dry composition of claim 17; and combining the dry composition with at least one of a binder and a solvent to form the slurry composition.
 19. An investment casting mold comprising: a prime layer; an intermediate layer disposed on the prime layer; a backup layer disposed on the intermediate layer; and a seal layer disposed on the backup layer; wherein at least one of the prime layer, intermediate layer, backup layer, and seal layer comprises a slurry composition comprising a refractory material, glass bubbles, and a thixotropic agent comprising fibrillated fibers.
 20. (canceled) 